1. Field of the Invention
The present invention relates to an image forming apparatus, such as a copier, an MFP (multifunction printer, product, or peripheral), or printer, that forms an image by printing on, while conveying, a sheet of paper.
2. Description of Related Art
For example, in an image forming apparatus, such as a copier, an MFP, or a printer, sheets of paper fed from a sheet cassette or the like are conveyed inside the apparatus, are printed on, and are ejected out of the apparatus. During conveyance of a sheet, skewing may occur. Factors causing skewing include how sheets are placed in a cassette, how conveyance rollers and the like for conveyance of sheets are fitted, and secular changes due to wear in the conveyance rollers. A sheet in a skewed state is easily caught in a conveyance passage, or at a rotary member (such as a pair of conveyance rollers) along a conveyance path. When that happens, a jam (getting-stuck of a sheet) results. Thus, some image forming apparatuses are devised to reduce skewing of sheets.
For example, there is known a sheet conveying apparatus comprising: guide members disposed to face opposite edges of a sheet in the direction perpendicular to the sheet conveyance direction and movable between a position for guiding the sheet and a position for aligning its edges; a reference sensor capable of detecting the initial position of the guide members; and driving means for driving the guide members, the guide members being moved according to size of the sheet, wherein there is provided a controller of which the input is connected to sheet orientation detecting means, disposed on the upstream side of the guide members, for detecting skewing of the sheet traveling into the interval between the opposing guide members and of which the output is connected to the driving means for driving the guide members, and the controller, on detecting that the skewing of the sheet is excessive, i.e., out of a predetermined state, retracts the guide members outward from a position ready to receive the sheet. With this construction, it is possible, basically, to align the edges of a sheet to correct skewing and, when a sheet is en excessively skewed state, to prevent its collision with the guide members and thereby prevent damage to the sheet and to the sheet receiving part as may result when the sheet collides with the guide members.
It is true that, with the conventional sheet conveying apparatus described above, the guide members for aligning the edges of a sheet can correct skewing to a certain degree; however, when a sheet has a large amount of skewing, the guide members are retracted simply to prevent damage to them, and a jam eventually occurs. Even when the guide members are moved, such a jam as occurs when a sheet has a large amount of skewing tends, invariably, to be severe, resulting in the sheet being torn, being stuck in a way difficult to take out, or remaining inside the apparatus. Thus, the conventional technology has the disadvantage of being incapable of preventing a severe jam.
Moreover, the conventional technology requires arrangement of a skewing correction mechanism (such as guide members; driving means such as a motor, a gear, a guide rod, and a belt; and a plurality of sensors) along the conveyance passage. This skewing correction mechanism is complicated and expanded, and thus has the disadvantage of increasing the manufacturing costs of image forming apparatuses. In fact, such a mechanism has in practical terms almost no hope of being adopted in low-end models (low-price-range models). Moreover, for effective skewing correction with large-size sheets, the guide members need to be secured a sufficient movement stroke. This disadvantageously leads to larger apparatus sizes.
Furthermore, even though the conventional sheet conveying apparatus may reduce incidence of jams, once a jam occurs during simultaneous conveyance of a plurality of sheets inside the apparatus, as during continuous printing, the user always has to remove a plurality of sheets from the conveyance passage. Thus, disadvantageously, the aim there is not to alleviate the trouble of recovering from a jam, and the work itself for that, on the user's part.